Just a bit of info.....
The determination of the Caspian Sea as a lake or a sea is in dispute and under negotiation between the 5 littoral states bordering it. A previous agreement between the Soviet Union and Iran listed it as a lake but since the break-up of the USSR the new littoral nations have refused to acknoweldge the previous treaty. If the Caspian were a lake the resources (oil, gas, fishing) and area would be split evenly between the five but if it is a sea the borders would be drawn out from the borders of each nation which would mean Iran would have considerably less territory under their control. There are other concerns but this is the primary issue.

If you are using maximum annual area, then Tonle Sap in Cambodia (16,000 sq km) qualifies. It varies annually between this size in November (end of the wet season) and 2700 sq km in April (end of the dry season).

That second bullet point in the quiz description is an odd one... "but not Lake Maracaibo". Wouldn't it be better to state the reason Lake Maracaibo isn't included, instead of just randomly excluding it? I'm assuming it has something to do with the fact that it's tidally influenced?

Another item that might bear further review is the inclusion of sub-glacial Lake Vostok. While it has been proven that there is water down there, it's also covered by 13,000 feet of ice which will never melt. Well, that is to say, if it DID melt, then we'd have a hell of a lot bigger problems than ranking lake sizes, because there'd be an ocean in most of our backyards.
Incidentally, if water located permanently beneath a hard surface is to be included, then Salar de Uyuni in South America should be on there. Below a salt crust surface is a 10,582 sq. km pool of brine.

Please accept Nyasa and Niassa as alternatives for Malawi. It is called those by Tanzania and Mozambique respectively. Former claims part of lake - latter unquestionably owns part. And Nyasa is what all British school kids were taught in my (ancient) generation!

Despite its name the Caspian Sea is often regarded as the world's largest lake, but it contains an oceanic basin (contiguous with the world ocean until 11 million years ago) rather than being entirely over continental crust

I think you need to include a note about seasonal lakes not being included in the quiz. Lake Eyre (max area 9,500km2) and Lake Torrens (max area 5,745km2) are large enough to make in onto this list but rarely have water in them because they are both in Outback Australia.

I had no idea that someone actually went out to these desolate lakes in Canada (Athabaska, Great Bear, Great Slave,) and actually measured them, shows what kind of civil service we had umpteen years ago.